1. Field of the Invention
The present invention relates to biological detoxification of drain waters from high pH aqueous environments to remove high concentrations of soluble cyanides, thiocyanates, and toxic heavy metals.
2. Description of Prior Art
Heap leaching, in metallurgical mining terms, involves stacking crushed ore on impermeable liners. A cyanide solution is sprayed over this stockpile and permeates the ore particles. Cyanide causes gold, silver, copper, and other metals to be solubilized. Metals of economic interest are then recovered by conventional separation means.
Wastewaters from the cyanidation process contain high concentrations of free cyanide, metal complex cyanide, ammonia, and thiocyanate. These compounds form by chemical interaction between cyanide and sulfide which are commonly present in metal bearing ores. Cyanidation wastewaters are potentially toxic to aquatic organisms, wildlife, and human beings. Thus, cyanide complexes, heavy metals, and ammonia must be removed before discharge of these wastewaters into surface or ground waters serving as potential potable water sources, or into marine or fresh water habitats.
Several chemical treatment process for removal or detoxification of cyanidation process wastewaters have been employed. Conventional processes utilize ozonation; alkaline chlorination; chlorine dioxide; copper catalyzed hydrogen peroxide; sulfur dioxide/air; acidification, volatization, and reneutralization; evaporation; and sludge impoundment. These methods are limited in their capacity to remove highly concentrated cyanide on an economic basis.
Ozonation oxidizes free cyanides, weakly complexed metal cyanides, and thiocyanate, but fails to oxidize strongly complexed metal cyanides such as ferri- and ferro-cyanide. Ammonia, a by-product of cyanide or thiocyanide oxidation with ozone, is also not oxidized. At high levels, ammonia is toxic to humans and aquatic organisms. Ozone has limited solubility which results in poor treatment performance and a costly process.
Alkaline chlorination processes utilize chlorine compounds, e.g., chlorine gas, hypochlorite, or chlorine dioxide, which remove cyanides and precipitate metals at elevated pH. Iron complexed cyanides, ammonia, and chlorides are not removed. Thiocyanate oxidation may demand excessive chlorine and strict pH control is required for effective metals removal.
Copper catalyzed hydrogen peroxide processes, e.g., those described in U.S. Pat. No. 3,617,567 to Mathre, are becoming the preferred chemical process in the mining industry. These processes remove free and metal complexed cyanides through oxidation, including the ferri- and ferro-cyanides. However, both thiocyanate and ammonia remain. Metals are removed through precipitation. As with ozonation or chlorination, the large volumes of metal hydroxide sludges must be removed from the environment. The added copper catalyst is toxic to aquatic organisms at very low concentration and must be carefully removed before water discharge. Moreover, large amounts of expensive hydrogen peroxide are used, regardless of the cyanide concentration.
International Nickel Company's sulfur dioxide/air oxidation process, which is also copper catalyzed, is similar to other oxidative chemical processes. High sulfate effluent levels and large volumes of metal sludge are process drawbacks.
An acidification, volatilization, and reneutralization process utilizes strong acidic conditions to volatilize cyanides as hydrogen cyanide gas. The gas is trapped with a caustic soda solution. Metals are then concentrated into a hydroxide sludge. At present, the economics of this process are unfavorable in most situations.
Containment of wastewaters followed by evaporation is utilized in some arid areas. Residual sludges must be impounded or treated by other means as toxicity is concentrated from the solution into the sludge formed.
Biological treatment methods have been proposed for aerobic (requiring oxygen) and anaerobic (in the absence of oxygen) destruction of cyanides. The microorganisms Alcaligenes and/or Achromobacter were added to activated sludge to degrade nitriles and cyanides, see U.S. Pat. No. 3,756,947 to Fujii et al. U.S. Pat. No. 3,940,332 was issued to Kato et al. for the use of Nocardia to remove nitriles and cyanides from wastewaters. The genus Pseudomonas has produced species capable of decomposing hydrogen cyanide (U.S. Pat. No. 3,660,278 to Mimura). Multiple stage combinations of chemical and biological treatment systems are disclosed in U.S. Pat. Nos. 3,816,306 to Roy, and U.S. Pat. No. 4,188,289 to Besik.
Pseudomonas paucimobilis, see U.S. Pat. No. 4,461,834 to Mudder and Whitlock, is used in a full scale treatment facility as described in U.S. Pat. No. 4,440,644 to Mudder and Whitlock. That biological facility treats low concentration metal complexed cyanide wastewaters at near neutral pH. The bacterial strain Pseudomonas paucimobilis is described in Holmes et al. (1977) Int. J. Sys. Bacteriol. 27:133-146.
However, none of these methods provide an effective and economical means for treating heap leach pad wastewaters. These wastewaters are characterized by particularly high cyanide concentrations and high pH. The present invention provides methods and means for economical treatment of these highly polluted waters.